1. Field of the Invention
The present invention relates generally to a valve timing control apparatus which controls the timing of at least one of the intake valve and exhaust valve of an engine. More particularly, this invention relates to a valve timing control apparatus which controls the amount of the valve overlap of the intake valve and exhaust valve in accordance with the running condition of an engine.
2. Description of the Related Art
In the conventional combustion engine, intake and exhaust valves respectively operate to open and close air-intake and exhaust passages connected to the individual combustion chambers. The timing of both valves, likewise that of the pistons, is controlled by the rotational phase of the crankshaft. As shown in FIGS. 8(a) and 8(b), during a cycle where the crankcase angle changes 720 degrees, a short period of time exists when both valves of a cylinder open at the same time. This so called valve overlap helps to scavenge burned gases out of the cylinder and to pull fresh air and fuel into the cylinder. Consequently, the amount of intake or exhaust gas in any one of the combustion chambers depends on the angle of a throttle valve provided in the engine's air-intake passage, or more specifically, on the speed of the engine at any particular time.
Various apparatuses are available to ensure a variable valve timing in order to control the amount intake and exhaust amounts in the combustion chamber with a greater degree of freedom. Japanese Unexamined Patent Publication No. 58-25538 discloses one example of such an apparatus. As illustrated in FIG. 9, an injector 92 provided in an air-intake passage 93 of an engine 91 injects fuel into the passage 93. An intake valve 96 and an exhaust valve 97 respectively open, allowing communication between a combustion chamber 94, the air-intake passage 93 and an exhaust passage 95. Actuators 98 and 99 drive the valves 96 and 97 respectively. A computer 100 controls the driving of the injector 92 based on various parameters detected by individual sensors 101, 102 and 103, and in this way adjusts the amount of fuel injected into the air-intake passage 93. Based on various engine operating parameters, the computer 100 also controls the driving of the individual actuators 98 and 99. This in turn effects the timing control of the valves 96 and 97.
Through the above control, the computer 100 controls the adjustment of the air-fuel mixture, supplied to the combustion chamber 94, and thereby controls the output of the engine 91. Under conditions when the engine's speed is substantially constant, the computer 100 controls the overlap of the intake and exhaust valves 96, 97 in such a way that allows exhaust gas to recirculate into the combustion chamber 94. The purpose of such Exhaust Gas Recirculation (EGR) is to reduce the amounts of nitrogen oxide contained in the exhaust gas exhausted to the atmosphere. In conventional engine systems, such control could also be carried out based on engine load rather than engine speed.
Usually, however, when the engine 91 has a small load, the amount of air fed into the combustion chamber 94 is itself relatively small, resulting in unstable combustion of the air-fuel mixture in the combustion chamber 94. Therefore, unburnt gas is likely to be left over after combustion in the combustion chamber 94. This tends to increase the level of engine emissions, i.e., exhaust emissions. A simple increase of the valve overlap to enhance the effect of the internal EGR operations increases, in this situation the amount of exhaust gas blown back to the air-intake passage 93 from the combustion chamber 94. Should too much exhaust gas be blown back into the air-intake passage 93, unstable air-fuel combustion occurs resulting in engine misfirings and increases to the engine's emission output. Even with intermediate engine loads, setting the valve overlap to a relatively large valve or amount often causes engine misfirings and subsequent increases to the amounts of engine emissions exhausted to the atmosphere. This is the case irrespective of whether the air-fuel mixture is burned properly in the combustion chamber 94.